Seismotectonic Changes before the M = 9.1 Sumatra Earthquake of December 26, 2004

Abstract—The vertical movements measured by broadband seismic stations located around the epicenter of the December 26, 2004 Sumatra 2004 mega-earthquake of magnitude 9.1 are analyzed. It is shown that during the five years before the earthquake (1996 to 2000), the COCO station closest to the epicenter, located 1700 km away, recorded quiet daily variations. Step-like distortions in the level of the recorded seismic noise appeared at this station in 2001 and continued up to the time of the earthquake. The station has also recorded pulses lasting a few minutes, with amplitudes gradually increasing to several times the amplitude of diurnal variations, followed by a decrease. The pulses occurred under quiet meteorological and geomagnetic conditions. No such pulses were found in the records of stations more than 2000 km from the epicenter. It is hypothesized that the sharp changes in the low-frequency seismic noise reflect tectonic slips on the faults in the lithosphere of the Indian Ocean.